RPC changes
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- The `walletcreatefundedpsbt` RPC call will now fail with
  `Insufficient funds` when inputs are manually selected but are not enough to cover
  the outputs and fee. Additional inputs can automatically be added through the
  new `add_inputs` option.

- The `fundrawtransaction` RPC now supports `add_inputs` option that when `false`
  prevents adding more inputs if necessary and consequently the RPC fails.

- A new `send` RPC with similar syntax to `walletcreatefundedpsbt`, including
  support for coin selection and a custom fee rate. The `send` RPC is experimental
  and may change in subsequent releases. Using it is encouraged once it's no
  longer experimental: `sendmany` and `sendtoaddress` may be deprecated in a future release.